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The Environment / Re: What was the early Earth atmosphere like?
« on: 17/06/2018 00:45:57 »There is one very important thing about the early (actually not so early) atmosphere, that is usually being forgotten. Keep in mind that insects were much larger the farther you go back in "history". The only reasonable way to explain this, is by assuming a much denser atmosphere. Well, higher concentrations of O2 could have done that too, but they were never that much higher.The OP is asking about the primordial atmosphere, not the one that existed recently in the Carboniferous. Thus your post appears to be off-topic.
So a 100 or a 200 million years ago (and so on) we not just had higher concentrations of CO2, but also a multiplicator as there was much more atmosphere all over. Next to higher temperatures, and accordingly more vapour in the atmosphere, that meant a greenhouse gas bonanza.
That brings up the question, how the planet was not fried back then. Unless of course we question the GHE itself.
Meanwhile, back on the proto-planet, the original primordial atmosphere was removed either by the solar outflow in its T-Tauri phase, or as a consequence of the moon-froming impact. As noted by evan_au the subsequent atmosphere was probably delivered by comets (or asteroids), though a proportion certainly arose from mantle degassing. Last time I looked, debate continued as to whether or not the atmosphere was reducing and, if it was, to what extent. I'll look for a recent review paper and post a link.